[llvm-ar] Update error messages and tests as per latest preferred style

It updates two error messages under `performOperation` in the file
llvm-ar.cpp. Furthermore, it also updates tests that print out these
error messages: `llvm/test/Object/ar-create.test` and
`llvm/test/tools/llvm-ar/print.test`.

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D80846